Radiation Shields and Ion Drives

[Aireal]

Could Ion Drive technology be the key to radiation shielding, while at the same time improving ion drive efficiency?

The standard ion drive has a stage of operation where it collects excess electrons and injects them into the ion beam to prevent the spacecraft from accumulating a large negative potential.

What if that stage could be eliminated, and the negative potential be used to help shield the crew and equipment. First we must look at the problems of radiation shielding for crews in outer space. The three main schools of thought on these are: Material Shielding, which has the drawback of mass. A major factor for a ship with ion drive. Magnetic Shielding, which is weak at some points, much like the Earth is. It also requires a strong magnetic field of about 20 teslas, which might have its own ill effects, we don't know yet. Electrostatic Shielding, by giving the hull a positive charge of 2 billion volts or so , you can repel cosmic-ray protons. But it creates a bombardment of electrons in the range of its field effect.

The key might be to combine the strengths of each of these, while reducing their drawbacks, with a unique hull concept.

Picture the hull as a giant capacitor, and the ion drive giving it a charge with the electrons that would normally be collected and ejected into the positive ion stream. Each charged layer of the hull would be separated by an insulator material that is also a good radiation shielding material. The most likely bet would be a plastic, or maybe a ceramic. The skin of the hull should have a neutral charge, where as one end of the ship would have a positive charge, and the other a negative, powered by the hull capacitor system. This will generate a magnetic shield around the ship. Now all three of the main shielding systems are in place, let's look at how they interact.

At this time a nuclear electric propulsion (N.E.P.) can deliver power levels of several hundred kilo watts to an ion drive, and may reach the megawatt category in the near future. This means an ion drive could generate a large charge in the hull to create a magnetic shield around the ship. I am not sure what the max. field strength would come to, but most likely not large enough to stop the radiation on its own. The good new is, it does not have to stop it, just redirect a few particles toward the poles of the ship, and slow down the rest.

Once the charged cosmic particles have been slowed down by the magnetic field, they will encounter the hull. Passing through the skin of the outer hull, they will next encounter the material which serves as insulator and shielding. This will slow the particles down even more. Then it will encounter the first charged layer of the hull capacitor system. These charged layers will act as electrostatic shielding within the hull. Then the cosmic particles must repeat the process of going through material and charged layers till it is hopefully stopped. Because our hull has both positive and negative layers in it, it will protect against a greater range of charged particles than the standard electrostatic field design. To a charged particle, this process will act as if the hull was a material shield with electrostatic shields within it, combining these two shielding concepts. As this setup is used to create the magnetic field, all three shielding concepts are now one system. So we now have all three of the main radiation shielding systems in place, all working together. So what are the advantages of this approach, let us look.

The first advantage is an overall reduction in mass needed to protect the crew. Every gram of weight saved in material shielding would be worth its weight in gold for an ion drive ship. With just material shielding alone, it would take about one kilogram of material per square centimeter to protect a crew, that's a lot of mass. Where as magnetic and electrostatic systems have huge energy requirements. By combining the magnetic and electrostatic systems into one, we have already cut the power needed by half. With the system using the potential difference created by the ion drive, the need for its own power supply is reduced or eliminated, another reduction in weight. Because the load of protecting the ship is split between three systems, we do not have to use as much power for magnetic and electrostatic systems as if the were run alone. So the overall energy requirements for the ship is reduced also. Yet every design has some drawbacks, so let's look at how these can be reduced in this system.

Magnetic fields provides little shielding at the poles. As our magnetic field is created by the charged areas at each end of the ship, this problem will be increased from particle bombardment in these regions. The simplest way to solve this is to place the crew area in the center of the ship, away from the poles. Antenna or other extensions from the ship could move the poles even farther away from the crew, for an added measure of safety. As the crew would also need shielding from the reactor, the ship design may look something like this. A long cylinder with a antenna projecting from the front of it, the reactor and ion drive at the rear, with the crew area in the center. Of course other designs are possible with this concept. The standard electrostatic shielding concept has the drawback of causing particle bombardment due to its field radius in space around the ship. By locking these charged areas in the hull with a capacitive design, we limit the range that this field expands into space, thereby reducing the problem of particle bombardment. Next we must pick the shielding/insulator material. As it must fill two jobs, it may take some research to determine the best choices. Plastics like polyethylene might be a good starting point. If a lightweight material for the charged layers could be found, this would help in mass reduction, and make the charged layers better as material shielding. Work is already being done in this area. Adding carbon, graphite or nanotubes to plastics is one approach, polycarbonate might be a good chice for this, as it would add some impact protection also. Interpenetrating Polymer Network ( I.P.N.), and similar work by companies like the Eeonyx Corporation are promising also. Thus we can reduce the disadvantages of each of these systems while taking advantage of their strong points. The next question is, are there any other aspects we need to address with this design concept?

What if the capacitive charge in the hull started climbing too high for some reason? There would need to be a system in place to bleed off excess power and divert it to the ion drive or some other use. In the event of a solar storm, primary power could be diverted to increase the field strength. After the danger has passed, the extra power could be sent to the ion drive. This approach may allow for less than max. protection in the normal state, and protection increased when needed, resulting in an even lighter ship design. Perhaps a Faraday Cage (RF shielding) could be constructed around the crew area to help protect them from the magnetic field effects also. Another area for improvements would be the outer skin of the hull. Could it be made to generate usable power from the constant bombardment of particles. A recent discovery by researchers of the Material Sciences Division of Berkeley and partners show that alloys of indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight, from the near infrared to the far ultraviolet, to electrical current. So even the outer hull of the ship may have more than one use in the future. The hull should be modular in construction and design. This will lessen the chance of system failure if a hull section gets damaged, allow repairs to be conducted in flight, and make construction quicker and more cost effective.

An increase in ion drive efficiency might be obtained by correctly locating the positive pole at the rear of the ship in relation to the ion drive. As the positive ion stream left the ion drive it would encounter the positively charged field at the rear of the ship. The ion stream would be repelled even faster away from the ship, while imparting a slight increase in thrust to the ship. While it may not be much of an increase, every little bit helps over long interstellar trips.

Hopefully this approach would reduce the weight and energy requirement of our spaceship by 2/3 over standard designs, while still protecting the crew and equipment. I have been working on this for some time and would look forward to any comments. A fresh outlook on this may help me detect problems I am unaware of.

[Nik]

Hi, welcome to forum !!

A neat combo, and *certainly* food for thought, but I think there's a problem with higher-energy and/or neutral components that may not 'notice' the electric field...

I suspect that charging the layers as a 'capacitor' may not help: what if secondaries are accelerated ? Sorry, I don't have the math...

There's also the mass-penalty of cladding entire ship. Given that craft will have fair warning of flares, swinging ship to put the reactor shielding 'up-wind' of 'storm cellar' must be first priority.

Then, you can mount additional material on the reactor's truss to improve the shielding. Perhaps arrange your charged layers in a steep cone ?? Think of the grazing-incidence construction of x-ray and soft-gamma telescopes, but 'divergent'. There's still the issue of 'Penumbra' as Sun is an extended source...

IIRC, Coronal Mass Ejections will long remain the equivalent of Mariners' 'Rogue Wave'-- 'Overwhelmed By Sea'.

[Aireal]

Hello Nik

Thank you for giving me some food for thought. I was staggered by the sheer mass and/or power requirements needed for this kind of undertaking. I soon discovered that I too lacked the math. I glossed over some material in the post, as it was too long any way. I know I can not take this much farther myself. I suspect that there may be a number improvements could be made, so long as the base concept is sound. Again, thank you for the post, I must ponder some more.

[DonPMitchell]

You could allow the ion engine to charge the hall.  That will not create a magnetic field, but it will create a negative electric field.  Unfortunately, the effect of that will be to accelerate cosmic rays toward the ship with even greater energy, and also it will diminish the velocity of the ion engine exhaust, by pulling the ions back toward the ship.

A magnetic shield idea is good, and people have looked into that.  To create a magnetic field around the ship, you need to crate an electric current, but that can be done in a super-conducting magnet and not consume much power after it is initially powered up.

Here's an article: http://www.thespacereview.com/article/308/1

[Aireal]

Hello Nik, here is a link to a scientist who is conducting research into stopping radiation using the electrostatic field of a charged capacitor. www.signaldisplay.com/electric.html
Someone from another forum told him about my concept, add he sent me the link to his site. From what he and other scientists I have talked to said, it seems I am at least on the right track. My post was quite long, and I glossed over some details to keep down the length. That seems to have been a mistake, as I am constantly having to clear up the points I glossed over.
I did not plan on shielding the entire ship, but as this was only part of a larger ship design, I left out details I should not have, sorry about that.

Hello DonPMitchell, Not all of the ships hull is charged like a convental electrostatic shield design. The ship is a large dipole, and the dipole is used to create the magnetic shield. Sorry about that, my post was not as clear as it should have been.

[Satanic Mechanic]

Another look in my opinion.
Instead of using a magnetic field to repel the particles, some materials can be used to repel, slow down and absorb radiation.  Take an approach to build the spacecraft like a nuclear reactor.  I am not saying we encase the ship with a two foot Lead hull, just use a smart layout for radiation protection. The whole ship does not need to covered.
Put the water storage tanks facing out, water is great at slowing down neutrons (yes I know cosmic rays are mostly energetic protons) and so is graphite and both can be used as a deflector.  Use a boron layer as the absorber.

SM

[DonPMitchell]

Aireal, a charged dipole does not generate a mangetic field, it makes an electric field.  I'm still not quite sure how this works.

A magnetic field doesn't slow down cosmic rays, it just deflects their direction somewhat -- in the direction perpendicular to the magnetic field and the ray's velocity.  An electric field would also deflect the particles toward a negative electric pole.

The question is, deflect by how much.  So you need to calculate that, from a field strength and the average cosmic ray velocity.  If the rays are just bent slightly, then the field won't really make any difference.

[ijuin]

If the rays could be deflected sideways by as little as say, ten meters, then it would be possible to deflect them around the hab module since it would create a small zone of relative safety.

[DonPMitchell]

There's been a lot of work about on magnetic deflection, creating an artificial van-Allen belt around the spacecraft.  I think that is simpler, using super-conducting magnets, than trying to create a power electric field.

[Aireal]

Satanic Mechanic,
I also use material shielding as part of the hull design, but it takes a lot of material shielding to have an noticable effect, 5 meters of water would be needed to provide the min. level of protection. Still, your idea of putting the water storage tanks facing out will help. As water is needed anyway, why not get more use out of it by useing it as part of the shielding? Sounds good to me. I was considering using graphite as a conductive layer in the hull, but was unaware that boron is a good asorber also, I will have to look into that. Thanks.

DonPMitchell,
You are correct that a charged dipole at a steady state does not produce a magnetic field. A dipole thats potential is changing will produce a magnetic field only during that time, much like a dipole tramsmitter antenna that does this by going in and out of phase. My first design used a coil in the hull to generate the field, but when I realized that the dipole would generate this field as it is in a constant state of flux, I removed the coil to simplfy the design. I hope I was correct in this choice.
You are also correct that a magnitic field does not slow down cosmic rays, rather it deflects them. It also lowers the rays energy level, which is what I meant when I said that it slowed them down. Sorry, I could have used better wording in my post, and that is one of the reasons I was wanting feedback. I must first be able to present my idea in a clear manner for it to be understood, and english is not my strong point.
You noted that there is a lot of work on magnitic deflection using superconducting magnets. All of these approaches would cost a great deal more than mine to implement, they need to generate a far greater magnitic field, and have larger energy requirments. They are effective, but I was aiming for a cheaper way to achive the same results.

Ijuin,
I see you noticed one of the more interesting aspects of my concept. First the cosmic radiation is deflected towards the charged poles of the ship. Columb attraction from the charged poles aid this, and becomes stronger as particles approach the charged poles. All of this helps to direct radiation away from the center of the ship where crew and equipment will be. The charged particles impacting the poles act as a varible load on the system, helping the dipole to remain in a constant state of flux. Radiation that does impact the hull must penatrate the material shielding and the charged layers within it which act as small electrostatic shield layers. Very perceptive of you, even though I have posted this on a number of forums, you are the first to notice this. All of the systems in my concept are interelated, and must be viewed as a whole to fully understand how they work. I have been working on this for 5-6 years now, so things that I think are apperant, may not be, as I have learned. Plus I am no expert in this field, and I often do not explain myself clearly to those more educated than myself. I am a retired stage hand, and only have a two year degree, though I have taken college courses that interested me throughout my life. That means that there are gaps in my knowledge that someone with a more formal education would not have. Which is the other reason for posting my concept.

[DonPMitchell]

Aireal: A magnetic field does not alter the energy of a cosmic ray, it just changes its direction.

Ijuin: The problem with a small deflection is that you might deflect a ray 10 meters to miss the hull...but then you will also deflect a ray that would have missed by 10 meters so it does hit the hall.

As I understand it, the magnetic field of the Earth is able to deflect incoming solar protons and electrons until they curve completely around into orbits.  They are then trapped in helical trajectories that may eventually bring them to collide with one of the poles of the planet.

[ijuin]

Ijuin: The problem with a small deflection is that you might deflect a ray 10 meters to miss the hull...but then you will also deflect a ray that would have missed by 10 meters so it does hit the hull.

No--the field strength drops off with the square of distance, so the number of rays that are pulled the right distance towards the magnets that they will hit the crew cabin would be something like four times less than the number that are pulled AWAY from the crew cabin to begin with.

[DonPMitchell]

Magnets do not pull rays toward them, they deflect the rays at right angles to the lines of force.  A dipole field also drops off as 1/r**3.  I don't think a ray will miss the hull unless it is bent strongly around before it hits the hull.  Just googling around, here's a 1991 paper on magnetic shielding, and he also mentions the idea of an electrostatic field:

http://www.islandone.org/Settlements/MagShield.html

He cites a paper from 1961, so this is an old idea.

[Aireal]

DonPMitchell

I may be wrong, but I assumed that some engery would be lost by any object that was deflected from its path, as I was taught in basic Newtonion physics. If you can explain why this is so, it would be of great help, and may cause some design changes. As for the path of a particle needing to be bent strongly around the hull, this would only be the case for particles approaching from the "wrong" side of the ship. Example, a proton approaching from the postive rear of the ship. Protons approaching from the other direction would encounter the negative pole and be attacted to it, never reaching the magnitic field before capture. I did look up the post you mentioned, and yes it is old, but it does still provide relavent info for those unfamilier with this topic. I was surprised that it suggested that Con-centric spheres of opposing charges be used for solving some of the problems with electo-static shielding, highly ineffective and only addresses one of the problems with that concept.

Ijuin

Your projection of a 4x reduction in rays hitting the crew area are inline with what I calculated.

[DonPMitchell]

A magnetic field does not attract or repel a charged particle, it only changes the direction of a moving particle.  The kinetic energy of that particle remains the same.